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Sci Tech
Keeping fibre-optic signals sharp
Optical fibres are used increasingly nowadays to carry information in the form of pulses of light over vast distances. More and more of communications — from text messages to high-definition television — travel over optical fibre.
But there is a hitch: Every 50 miles or so the signal must be reamplified, cleaned up and relaunched. Light is dimmed by miles of fibre, and the crisp on-and-off pulses that represent the ones and zeros of a digital signal become misshapen and fuzzy.
Now Cornell University researchers have demonstrated that all this can be done on a single photonic microchip, according to a Cornell University press release. Previously the researchers had demonstrated a light amplifier on a silicon chip using a process called four-wave mixing, which could amplify an optical signal by ‘pumping’ with another beam of light.
Four-wave mixing has been used to amplify light in devices made of optical fibre, but the process requires tens of metres of fibre.
The Cornell researchers used silicon waveguides only a few hundred nanometres across and 1.8 centimetres long embedded in a single silicon chip.
The tight dimensions of the waveguide, smaller than the wavelength of the light travelling through it, forces two entering beams of light — the signal and the ‘pump’ — to exchange energy. Some photons from the pump are converted to the same wavelength as the signal, amplifying it, while others come out at a wavelength equal to twice the pump wavelength minus the signal wavelength. That last effect can be used to convert a signal from one wavelength to another.
The researchers found that pumping a pulsed signal with a continuous wave light beam at another frequency amplifies the signal but doesn’t clean up the pulses.
However, if the arrangement is changed so that the light carrying the signal acts as the pump, the output is both amplified and sharpened. — Our Bureau
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